Such a cleaning assembly generally is arranged in the combine harvester below a threshing and/or separation unit for receiving crop material that is to be subjected to a cleaning operation, such as for example threshed grain from which chaff and other impurities need to be removed in order to retain cleaned grain. The cleaning assembly normally comprises a fan for generating a flow of air that cooperates with a sieve assembly comprising reciprocating sieves longitudinally extending along the direction of the crop flow. The sieves comprise openings such that when crop material is received on them, the clean grain can fall through these openings and the airflow can pass these openings for blowing impurities and chaff towards the downstream end of the sieves. The clean grain is then accumulated below the sieves at a clean grain auger from where it is for example subsequently transported to a grain tank of the harvester.
WO8100503A1 (Massey Fergusson, 1981) shows a harvester comprising a cleaning assembly comprising a plurality of fans of which the speed can be independently controlled. One of the fans 12 provides an airflow to the sieve assembly 11 shown in the Figure of WO8100503A1. Another fan 10 provides an airflow at an outlet directed at an opening that forms a fall step for the crop between the feeder 9 and the grain pan 7. It is clear that the distribution of the airflow, towards the sieves and also the local air pressure when viewed along the driving direction varies. This means that closer to the outlet of the fan 12, at the upstream side of the sieves 11 with respect to the crop flow, the airflow and air pressure will be higher than further away from the outlet at the downstream side of the sieve. The same holds for the distribution of the airflow and air pressure generated by other fan 10 with respect to the grain pan 7. The grain pan 7 not being permeable for this airflow still further enhancing this effect.
WO2005/018303A1 (Straeter, 2005) shows in its FIG. 8 an embodiment of a combine harvester that comprises two fans providing an airflow near the upstream side of the sieves of the cleaning assembly. Both fans can be controlled independently, one fan is directed to the lower sieve, the other is directed to both the upper and the lower sieve. Since the outlet of both fans is nearly at the same location with respect to the direction of the crop flow the distribution of the airflow and air pressure along the sieves will be higher than the airflow and air pressure further away from the outlet at the downstream side of the sieve.
EP2476304A1 (Claas, 2012) describes a combine harvester with a plurality of fans arranged along a direction transverse to the crop flow at the upstream side of the sieves. The blowers can be controlled independently of one another. The independent control of the air volume flow output by the fans makes it possible to vary the air flow supplied to the sieve system so that can vary across the width of the sieve system, for example, to compensate an uneven distribution of crop material on the sieves on a slope transverse to the driving direction. Although this system provides control for the distribution of the airflow towards the sieves in a direction transverse to the crop flow, which is generally referred to as the transversal direction of the cleaning system generally transverse to the longitudinal direction of the cleaning system. Furthermore the cleaning system requires a large grain pan arranged below the threshing unit and covering the distance to the outlet of the fan. In this way the area available for elements most actively involved in the cleaning operation such as the sieves of the cleaning system is reduced.
U.S. Pat. No. 4,259,829 (Sperry, 1981) describes a combine harvester comprising a cleaning system with a single fan arranged near the upstream side of the sieves. Adjustable baffles are provided in order to influence the distribution of the airflow along the longitudinal direction of the cleaning system in response to sensors detecting the distribution of the crop material on the sieves along both the longitudinal and transverse direction. However these baffles affect the efficiency of the fan system as they form an obstruction to the airflow generated by the fan. Furthermore these baffles only allow for a limited modification to the distribution of the airflow along the longitudinal direction of the sieves as in such a setup the airflow will generally always decrease towards the downstream end of the sieves.
EP2550852 (Claas, 2013) also discloses a sensor system capable of measuring the distribution of the crop material on the sieves along both the longitudinal and transverse direction.
There still remains a need for an improved cleaning assembly for a harvester that is able to augment flexibility and efficiency in controlling the cleaning operation along the direction of the crop flow and is able to make more efficient use of the available area for active cleaning elements such as for example the sieve elements which are most actively involved in the cleaning operation.